This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. There are more than a dozen Ubiquitin-like proteins (Ubls) in higher eukaryotes (e.g. ubiquitin, NEDD8, SUMO, ISG15) that covalently modify myriad substrates to alter the functions of their targets in different ways. Ubls are covalently attached to targets by parallel, but Ubl-specific enzymatic cascades involving E1, E2, and E3 enzymes. These enzymes generally comprise multi-protein assemblies, and over 700 human proteins have motifs indicating that they are part of E1, E2, or E3 enzymes. Although the physiological functions are known for only a handful of these proteins, in the past few years defects in proteins involved in Ubl conjugation have been associated with diseases such as cancer, neurodegenerative disorders, and viral infections. We are determining the structures of selected E1s, E2s, E3s, complexes with Ubls, and complexes with targets in order to obtain a general molecular picture of catalytic mechanisms underlying Ubl conjugation.